avoid computing liveness for locals that escape into statics

[nikomatsakis]

Fixes #52713

I poked at this on the plane and I think it's working -- but I want to do a bit more investigation and double check. The idea is to identify those local variables where the entire value will "escape" into the return -- for them, we don't need to compute liveness, since we know that the outlives relations from the return type will force those regions to be equal to free regions. This should help with html5ever in particular.

• test performance
• verify correctness
• add comments

r? @pnkfelix
cc @lqd

[nikomatsakis]

@bors try

[bors]

⌛ Trying commit 341a07c with merge da90b26...

[bors]

☀️ Test successful - status-travis
State: approved= try=True

[nikomatsakis]

@rust-timer build da90b26

[rust-timer]

Success: Queued da90b26 with parent 03da14b, comparison URL.

[nikomatsakis]

@bors try

[bors]

⌛ Trying commit 75504ee with merge 24a313ef05eff74c552335159aa02fbbfcf7e8b3...

[nikomatsakis]

without support for X = &*Y reborrows, this won't work so well :)

[nikomatsakis]

@rust-timer build 24a313ef05eff74c552335159aa02fbbfcf7e8b3

[rust-timer]

Success: Queued 24a313ef05eff74c552335159aa02fbbfcf7e8b3 with parent 7e8ca9f, comparison URL.

[bors]

☀️ Test successful - status-travis
State: approved= try=True

[nikomatsakis]

@rust-timer build 24a313ef05eff74c552335159aa02fbbfcf7e8b3

[rust-timer]

Success: Queued 24a313ef05eff74c552335159aa02fbbfcf7e8b3 with parent 7e8ca9f, comparison URL.

[nikomatsakis]

The comparison URL doesn't work yet but you can view the NLL dashboard results for 24a313e here:

https://perf.rust-lang.org/nll-dashboard.html?commit=24a313ef05eff74c552335159aa02fbbfcf7e8b3&stat=instructions%3Au

html5ever	11,036,800,000.00	241,127,686,144.00	2,184.76
tuple-stress	24,313,452,544.00	69,815,214,080.00	287.15
keccak	47,153,451,008.00	85,099,872,256.00	180.47
clap-rs	21,144,303,616.00	30,054,019,072.00	142.14
sentry-cli	12,483,297,280.00	15,415,211,008.00	123.49
inflate	13,858,590,720.00	16,881,270,784.00	121.81

versus

html5ever	11,049,374,720.00	359,718,027,264.00	3,255.55
tuple-stress	24,456,257,536.00	69,911,584,768.00	285.86
keccak	47,174,316,032.00	85,679,489,024.00	181.62
clap-rs	21,200,508,928.00	31,260,983,296.00	147.45
inflate	13,863,846,912.00	17,317,126,144.00	124.91

[nikomatsakis]

Seems to be a good win on html5ever, as predicted =)

[nikomatsakis]

@pnkfelix ready for review now I think

[pnkfelix]

@bors r+

[bors]

📌 Commit 5e2f337 has been approved by pnkfelix

[bors]

🌲 The tree is currently closed for pull requests below priority 12, this pull request will be tested once the tree is reopened

[rust-highfive]

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[kennytm]

@bors r-

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[00:13:14] �[0m  �[0m�[0m�[1m�[38;5;12m--> �[0m�[0mlibrustc_mir/borrow_check/nll/escaping_locals.rs:79:5�[0m
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[00:13:14] �[0m   �[0m�[0m�[1m�[38;5;12m= �[0m�[0m�[1mnote�[0m�[0m: `-D dead-code` implied by `-D warnings`�[0m
[00:13:14] 
[00:13:14] �[0m�[1m�[38;5;9merror�[0m�[0m�[1m: field is never used: `mir`�[0m
[00:13:14] �[0m  �[0m�[0m�[1m�[38;5;12m--> �[0m�[0mlibrustc_mir/borrow_check/nll/escaping_locals.rs:80:5�[0m
[00:13:14] �[0m   �[0m�[0m�[1m�[38;5;12m|�[0m
[00:13:14] �[0m�[1m�[38;5;12m80�[0m�[0m �[0m�[0m�[1m�[38;5;12m| �[0m�[0m    mir: &'cx Mir<'tcx>,�[0m
[00:13:14] �[0m   �[0m�[0m�[1m�[38;5;12m| �[0m�[0m    �[0m�[0m�[1m�[38;5;9m^^^^^^^^^^^^^^^^^^^�[0m
[00:13:14] 
[00:13:14] �[0m�[1m�[38;5;9merror�[0m�[0m�[1m: aborting due to 2 previous errors�[0m
[00:13:14] 
[00:13:14] �[0m�[0m�[1m�[31merror:�[0m Could not compile `rustc_mir`.
[00:13:14] 

[nikomatsakis]

@bors r=pnkfelix

[bors]

📌 Commit 2e2ea26 has been approved by pnkfelix

[nikomatsakis]

@bors p=1 -- NLL is Rust 2018 critical and this is a perf building block.

[bors]

⌛ Testing commit 2e2ea26 with merge b47c314...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing b47c314 to master...